Rocket motors employing solid propellants typically comprise a rigid outer casing or shell; a heat insulating layer (insulation) bonded to all or part of the inner surface of the casing; a liner layer (liner) bonded to the insulating layer; and a solid propellant bonded to the liner. The insulation is generally fabricated from a composition capable of withstanding the high-temperature gases produced when the propellant burns, thus protecting the casing (or selected parts of it). The liner is an elastomeric composition which bonds the solid propellant to the insulation and casing.
The known process of making a rocket motor typically comprises the following steps:
(1) cleaning and degreasing the inner surface of the rocket motor casing; PA1 (2) applying insulation to selected parts of the inner surface of the casing; PA1 (3) curing and baking out the insulation; PA1 (4) applying a bond promoter (primer) to the inner surface of the insulation and any exposed portion of the casing; PA1 (5) applying the liner over the entire internal surface of the casing, including the insulated areas; PA1 (6) partially curing (also known as precuring) the liner; PA1 (7) placing the lined motor in a propellant casting pit and heating the casing containing the precured liner to an appropriate temperature for casting; PA1 (8) casting the propellant into the lined, heated casing; and PA1 (9) fully curing the propellant and liner, generally by heating the motor assembly. PA1 a. providing a rocket casing having an internal surface; PA1 b. providing a blocked curable liner composition which is the reaction product of a prepolymer, an isocyanate curing agent, and a blocking agent; PA1 c. applying the liner composition to the internal surface of the rocket casing, forming a casing and uncured liner assembly; PA1 d. providing an uncured, castable propellant composition; PA1 e. preheating the casing and uncured liner assembly for receiving the propellant, which simultaneously unblocks the liner and precures it to a tacky state; PA1 f. casting the propellant into interfacial contact with the precured liner; and PA1 g. cocuring the liner and propellant compositions.
The prior art process has the disadvantage that steps (6) and (7) above must be performed separately. Step (6) has typically required a cure time of 16 to 24 hours at 170.degree. F. (77.degree. Celsius), and step 7 has typically required bringing the casing and liner to a temperature of about 145.degree. F. (63.degree. Celsius) to initiate the propellant cure as oon as the propellant is poured into the motor. Furthermore, once step 6 has been performed, steps 7 and 8 must follow in a short time to avoid overcuring the liner before the propellant is cast. Finally, because steps 7 and 8 are typically performed under vacuum and step 6 is not, step 6 is carried out in a different location to avoid tying up vacuum-equipped casting pits during a lengthy liner precuring operation.
Improvements in propellant processing taught in U.S. Pat. No. 4,110,135, issued to Graham et al., reduce the propellant cure time and increase its pot life. Thus, the propellant can be mixed, stored, and worked for relatively long periods of time without prematurely curing, but once the cure is initiated it proceeds rapidly to completion. Graham, et al. achieve these advantages by using a propellant binder system comprising hydroxyl-terminated polybutadiene based polyurethane (the reaction product of a hydroxyl terminated polybutadiene prepolymer and a polyisocyanate curing agent) and a curing catalyst comprising an organometallic compound and a carboxylic acid. (The carboxylic acid can optionally be formed in situ).
The same inventors, in U.S. Pat. No. 4,098,626, issued July 4, 1978, have disclosed that salicylate esters can reversibly block tertiary polyisocyanate curing agents which copolymerize with hydroxyl-terminated, polybutadiene based polyurethane propellant binders. The blocks are reversed by preheating the binder to its usual cure temperature, whereupon curing proceeds at a normal rate. But U.S. Pat. No. 4,098,626 teaches propellant binding compositions, not liners. One important difference between liner curing and propellant binder curing is that the latter operation is performed in a single step, while liner curing has required two steps (6 and 9 above). Another important difference between propellant and liner processing is that the temperature of a body of propellant cannot be rapidly and uniformly changed by changing the ambient temperature, while a liner is applied as a very thin coating on the exposed interior surface of a casing, so its temperature responds rapidly to changes in ambient temperature. The propellant art thus does not suggest the possibility of rapidly arresting or triggering a liner curing process by changing the ambient temperature. U.S. Pat. No. 4,098,626 is hereby incorporated by reference.